1. Field of the Invention
This invention relates to increasing efficiency of gas compression systems and more particularly to a system and method of using wasted heat energy from compression to increase the efficiency of the gas compression process.
2 Description of the Prior Art
Gas compressors have widespread application in industrial and domestic uses. Compressors provide a means of converting kinetic energy into potential energy. Kinetic energy is used to drive the compressor which in turn compresses the gas such that the compressed gas may be used at a later time to drive turbines or pneumatic tools, inflate tires, and for a variety of other applications. The kinetic energy is converted to potential energy and stored in the form of compressed gas much the way that a mechanical spring stores potential energy.
Unfortunately, the conversion process from kinetic energy to potential energy results in a waste of useful energy which leads to inefficiencies. As a gas is compressed, it is well known that the temperature of the gas increases as a result of the work of compression being done on the gas. This increased temperature can result in heat being lost from the compression system to the environment thereby wasting energy in the system.
In a compressor, there is an inlet pressure P.sub.1 and inlet temperature T.sub.1 of the gas prior to entering the compressor, and an outlet pressure P.sub.2 and outlet temperature T.sub.2 of the gas after compression. The theoretical interrelationship of these variables for ideal gas behavior is expressed by the relationship: EQU T.sub.2 =T.sub.1 (P.sub.2 /P.sub.1).sup.X
where X is a constant, expressed in terms of X=(.gamma.-1)/.gamma..
The term .gamma. is a the ratio of specific heat for a given gas, and both the inlet and outlet temperatures are expressed in terms of absolute temperatures. The work needed to compress an ideal gas for a given pressure ratio. P.sub.2 /P.sub.1, varies directly with the inlet temperature T.sub.1, and is expressed in the following relationship for ideal gas behavior: EQU .DELTA.H=C.sub.p T.sub.1 [(P.sub.2 /P.sub.1).sup.X -1]
where .DELTA.H is the work of compression for an ideal gas, C.sub.p is the specific heat at constant pressure of the gas, and the other terms are the same as above in the previous equation. Thus, if the inlet temperature is reduced, the work required to attain a given pressure ratio is reduced.
Furthermore, when the gas to be compressed has a high humidity level, additional inefficiencies result. When for example, energy is spent to compress humid air which later cools to a temperature at which the water vapor in the air condenses, the energy used to compress the water vapor is wasted. By dehumidifying the air (or other gas) prior to compression, the overall efficiency of the compression system is increased since less energy is wasted compressing water vapor which eventually would be condensed out of the air. Typically, such condensation occurs in storage where the compressed gas cools as heat is transferred from the hot, compressed gas to the surrounding environment.
The prior art discloses cooling a gas prior to compression in order to reduce the work needed to achieve a given pressure ratio. U.S Pat. No. 678,487 to Hill discloses an air compressor which pre-cools a gas to be compressed. This is done by having the pre-compression gas pass through an ordinary cooler having a coolant supplied by outside means. Also, the prior art in U.S. Pat. No. 706,979 to Martin teaches the use of simultaneous cooling whereby water jackets use an outside cooling source to remove heat from air prior to and simultaneously with compression. Also in U.S. Pat. No. 4,242,878 to Brinkerhoff simultaneous cooling is achieved by surrounding the compressor's compression chamber with liquid, thereby cooling the compression process.
Pre-compression cooling has also been extended to include inter-cooling where several compressors are in series or where there is multi-staged compression. U.S. Pat. No. 2,024,323 to Wyld and U.S. Pat. No. 4,554,799 to Pallanch both disclose the use of inter-cooling where two compressors in series are employed in a closed refrigeration system. U.S. Pat. No. 3,892,499 to Strub discloses a multi-staged turbocompressor with cooling between stages to cool air prior to second-stage compression, while reheating the air just prior to second-stage compression to vaporize any water droplet and thereby reduce condensate forming on the compressor blades.
At the post-compression end of the compressor the hot, compressed gas emerges. U.S. Pat. No. 4,279,574 to Kunderman discloses a system to heat buildings with the heat given off from the compressed gas. However, this system is of limited utility in that it is only useful where there is a building to be heated, and then only where climate requires such building to be heated.
In view of the foregoing, it would be a significant advance in the art to convert the post-compression heat into pre-compression cooling. In this way, the compression system could be made intrinsically more efficient, without dependence on external cooling systems or external heating requirements. However, it is counterintuitive to one skilled in the art of compressors to use heat to cool. Yet, that is a result which the present invention achieves. It would also be a significant advance to use post-compression heat to dehumidify the gas, rather than merely revaporize water droplets, prior to compression.